Mechanisms

In any reactor, a SCRAM is achieved by a large insertion of negative reactivity. In light water reactors, this is achieved by inserting neutron-absorbing control rods into the core, although the mechanism by which rods are inserted depends on the type of reactor. In pressurized water reactors (PWR), the control rods are held above a reactors core by electric motors against both their own weight and a powerful spring. Any cutting of the electric current releases the rods. A SCRAM rapidly (less than four seconds, by test) releases the control rods from those motors and allows their weight and the spring to drive them into the reactor core, thus halting the nuclear reaction (by absorbing neutrons) as rapidly as possible. In boiling water reactors (BWR) the control rods are inserted up from underneath the reactor vessel. In this case a hydraulic control unit with a pressurized storage tank provides the force to rapidly insert the control rods upon any interruption of the electric current, again within four seconds. A typical large BWR will have 185 of these control rods.

Liquid neutron absorbers are also used in emergency shutdown systems. During SCRAM the operators can inject solutions containing neutron poisons directly into the reactor coolant. Various solutions, including sodium polyborate and gadolinium nitrate, are used. Normally, these poisons are stored in pressurized tanks that are normally kept isolated from the reactor coolant system. When needed, valves or pumps are actuated to rapidly introduce the boron from these tanks into the primary loop. Because they may delay the restart of a reactor, these systems are only used to shut down the reactor if control rod insertion fails. This concern is especially significant in a BWR, where injection of liquid boron would cause precipitation of solid boron on fuel cladding, which would prevent the reactor from restarting until the boron deposits were removed.

Some modern naval nuclear power reactors have, in addition to scramming, the ability to automatically run the electric motors in reverse at high speeds for a few seconds, thus driving the rods into the core a short distance while leaving them latched to their motors. This "fast insertion" partially shuts down the reactor while leaving it ready to quickly restart—a consideration much more important in a warship than in a commercial power plant (also see Nuclear navy.)

Reactor response

Most neutrons in a reactor are prompt neutrons; that is, neutrons produced directly by a fission reaction. On average, these neutrons live for about 0.1 ms, which allows the insertion of neutron absorbers to affect the reactor quickly. As a result, once the reactor has been scrammed, the reactor power will significantly drop almost instantaneously. However, a small fraction (about .65%) of neutrons in a typical power reactor come from the radioactive decay of a fission product. These delayed neutrons will limit the rate at which a nuclear reactor will shut down.

Decay heat

On a SCRAM for a reactor that held a constant power for a long period of time (greater than 100 hrs), about 7% of the steady-state power will initially remain after shutdown due to the decay of these fission products. For a reactor that has not had a constant power history, the exact percentage will be determined by the concentrations and half-lives of the individual fission products in the core at the time of the SCRAM. The power produced decay heat slowly falls with the decay of fission products.

Etymology

When I showed up on the balcony on that December 2, 1942 afternoon, I was ushered to the balcony rail, handed a well sharpened fireman's ax and told that was it, "if the safety rods fail to operate, cut that manila rope." The safety rods, needless to say, worked, the rope was not cut… I don't believe I have ever felt quite as foolish as I did then. …I did not get the SCRAM [Safety Control Rod Axe Man] story until many years after the fact. Then one day one of my fellows who had been on Zinn's construction crew called me Mr. Scram. I asked him, "How come?" And then the story.

Other sources state that the term may actually mean super-critical reactor axe man, referring in that case to a person who would use an axe to cut a rope to drop a control rod into a reactor to shut it down. This became another meaning of the word SCRAM after people working at the first nuclear reactor pile in Chicago, Illinois, known as CP-1, incorporated it into their emergency procedures. (An alternative derivation is that it stood for Simulated Chicago Reactor Axe Man). Many attribute the usage to Enrico Fermi, who supposedly wrote the “axe man” phrase into the original reactor design. There were multiple safety systems in place at the Chicago pile, with some electrically-controlled control rods as well as vessels containing a cadmium solution available to stop any reactions if necessary. Therefore, the job of the “SCRAM” to drop another control rod by the force of gravity was most likely superfluous.

Other sources indicate that the term stands for safety cut rope axe man. The workers at CP-1 labeled an emergency shutdown button “SCRAM,” since they would immediately be scramming (running) from the premises (or to their emergency positions) as soon the button was hit. (In modern nuclear power plants, the operators do not leave the control room in the event of a SCRAM or even a major accident.)

[T]he safety rods were coated with cadmium foil, and this metal absorbed so many neutrons that the chain reaction was stopped. Volney Wilson called these "scram" rods. He said that the pile had "scrammed," the rods had "scrammed" into the pile.

An Atari 400/800 BASIC game called SCRAM simulated in a simplified way the operation of a nuclear power plant. The user manual stated that SCRAM stood for "Start Cutting Right Away, Man", referring to the rope that held the control rod in place.